Methods of improving adherence of emissive material in thermionic cathodes

ABSTRACT

An application of metal particles comprised substantially of nickel to a nickel containing cathode substrate is followed by the application of potentially emissive materials comprised of nickel coated carbonates of barium and/or strontium and/or calcium. The particles and nickel coated carbonates are diffusion bonded or welded to the substrate and to each other after the cathode is mounted in a tube, simultaneously with activation of the cathode. The coating thus provided is very adherent and reduces arcing during high voltage applications. An alternate embodiment for further increasing adherence of the emissive material comprises adding a nickel etching agent, such as barium nitrate, to the potentially emissive material suspension. During the temperature increase noted above to activate the cathode, the etching material first melts and densifies the carbonates and then decomposes to etch the nickel particles and the substrate. Further heating changes the etching material and stops the reaction.

United States Patent 1 Buescher [451 Mar. 27, 1973 [54] METHODS OF IMPROVING ADHERENCE OF EMISSIVE MATERIAL IN THERMIONIC CATHODES [75] Inventor: William E. Buescher, Emporium,

Pa. v

[73] Assignee: GTE Sylvania Incorporated, Seneca Falls, NY.

[22] Filed: June 30, 1971 21 Appl. No.: 158,576

[52] US. Cl. ..29/25.17, 29/2511, 117/217,

[51 Int. Cl ..H01j 9/00 [58] Field of Search ..29/25.l, 25.11, 25.14, 25.15,

[56] References Cited UNITED STATES PATENTS 2,172,207 9/1939 Kolligs et a1 ..313/345 X 2,943,957 7/1960 Grattidge et al.... ..l17/223 3,048,146 8/1962 Coppola ....1 17/223 X 3,110,081 ll/1963 Hendriks.... ....313/346 X 3,393,090 7/1968 Barraco ..l17/217 3,400,294 9/1968 Kling ..29/25.17

Primary Examiner-J. Spencer Overholser Assistant Examiner-Richard Bernard Lazarus Attorney-Norman J. OMalley et a].

[ 1 ABSTRACT An application of metal particles comprised substantially of nickel to a nickel containing cathode substrate is followed by the application of potentially emissive materials comprised of nickel coated carbonates of barium and/or strontium and/or calcium. The particles andnickel coated carbonates are diffusion bonded or welded to the substrate and to each other'after the cathode is mounted in a tube, simultaneously with activation of the cathode. The coating thus provided is very adherent and reduces arcing during high'voltage applications. An alternate embodiment for further increasing adherence of the emissive material comprises adding a nickel etching agent, such as barium nitrate, to the potentially emissive material v suspension. During the temperature increase noted above to activate the cathode, the etching material first melts and densifies the carbonates and then decomposes to etch thejnickel particles and the substrate.'Further heating changes the etching material and stops the reaction.

6 Claims, 1 Drawing Figure PREPARE FIRST VOLATILE SUSPENSION CONTAINING NICKEL PARTICLES PREPARE SECOND VOLATILE SUSPENSION CONTAINING PREPARE SECOND VOLATILE SUSPENSION CONTAINING NICKEL COATED CARBONATES NICKEL COATED CARBON- ATES AND NICKEL ETCHING AGENT SUCH AS BARIUM NITRATE SPRAY FIRST SUSPENSION ON NICKEL CONTAINING SUBSTRATE SPRAY SECONDSUSPENSION OVER FIRST SUSPENSION TEMPERATURE ABOVE CARBONATES PAIEIIIEIIIIARZTIEIB 722,045

PREPARE FIRST vOLATILE SUSPENSION CONTAINING NICKEL PARTICLES PREPARE SECOND vOLATILE PREPARE SECOND vOLATILE SUSPENSION CONTAINING 0R SUSPENSION CONTAINING NICKEL COATED NICKEL COATED CARBON CARBONATES ATES AND NICKEL ETCHING AGENT SUCH AS I BARIUM NITRATE SPRAY FIRST SUSPENSION ON NICKEL CONTAINING SUBSTRATE SPRAY SECOND SUSPENSION OVER FIRST SUSPENSION PROCESS COATED SUBSTRATE TAT TEMPERATURE ABOVE SINTERING TEMPERATURE OF NICKEL BUT B LOW MELTING TEMPERATURE TO DRIvE OFF BINDER AND wELD NICKEL PARTICLES TO SUBSTRATE, 'TO EACH OTHER. AND TO NICKEL COATED CARBONATE METHODS OF IMPROVING ADHERENCE OF EMISSIVE MATERIAL IN THERIVIIONIC CATHODES BACKGROUND OF THE INVENTION This invention relates to thermionic cathodes and more particularly to a method of increasing adherence of the emissive material to the cathode body or substrate to reduce arcing in high voltage applications and thus improve the performance and life of the cathodes and to cathodes produced thereby. The problem of arcing between the cathode and associated electrodes in high voltage vacuum tubes, with a concommittant peeling of the emissive material, has long been known and many solutions have been proposed to obviate the difficulty. One of the earliest proposals involved roughening the cathode surface to achieve better adherence of the emissive material. The roughening could be accomplished by sand-blasting or acid etching the cathode substrate before the application of the potentially emissive material. Another solution, proposed in 1937 by Kolligs et al. (see U.S. Pat. No. 2,172,207) contemplated applying a layer of powdered nickel or other suitable material to the cathode substrate in a parafiin oil carrier and then sintering the powder to the base. This left a roughened surface over which the potentially emissive material was applied. Still another solution was suggested by Toorks in 1945 (see U.S. Pat. No. 2,433,821). This solution involved fixing a wire mesh or screen to the cathode base and filling the holes therein with potentially emissive material. A still further solution was suggested by Hendricks in 1960 (see U.S. Pat. No. 3,110,081). Herein a cathode base had applied thereto a layer'of nickel particles or other suitable material which was then sintered. Then potentially emissive material was applied to the sintered layer and the excess removed. Subsequently, sufficient pressure was applied to the emissive material-particle layer to cold-weld the nickel particles to the base and to deform the particles so that the emissive material was retained in re-entrant cavities between the now deformed nickel particles.

While all of these approaches improved the performance of cathodes by increasing adherence of the emissive material, they suffered from disadvantages such as increased cost or the requirement of extra processing steps: viz., extra sintering. Additionally, all prior art attempts formed roughened surfaces or provided pockets in porous material to trap the emissive material without doing anything to the emissive particles per se. in the highly competitive field of vacuum tubes it would be advantageous if a cathode with good emission and a highly adherent coating could be made. Further, since any additional costs can be very detrimental it would be advantageous if a simple, inexpen-- sive method of achieving good emissive material adherence could be developed.

OBJECTS AND SUMMARY OF THE INVENTION It is, therefore, an object of this invention to obviate the disadvantages of the prior art.

it is another object of the invention to provide a new and novel cathode having a highly adherent emissive layer.

It is a still further object of the invention to provide a method of manufacturing such cathodes.

These objects are accomplished in one aspect of the The cathodes are produced by the provision of a method comprising the steps of applying a layer of powdered material, comprised of mostly nickel, in a volatile binder to a cathode substrate. A first spraying source is used for depositing the material. Directly over this layer is applied a layer of nickel coated emissive particles in a similar volatile binder. In an alternate embodiment this second binder contains a nickel etching agent that is non-reactive at room temperature. A suitable material is barium nitrate [Ba(NO After both the nickel layer and the nickel coated potentially emissive particles are applied, the cathode is completely processed within its associated tube. The processing involves raising the cathode temperature to about l,lOOC which simultaneously volatilizes the binders, activates and then changes the nickel etching agent (when it is used), welds the nickel coated emissive particles to each other, to the powdered material and to the substrate, and activates the emissive material. The processing temperature used is greater than the normal sintering temperature usually associated with nickel and less than the melting temperature thereof. This method eliminates the separate sintering step taught by g the prior art and provides a thermionic cathode with a strongly adherent emissive coating. The nickel coated emissive material and the etching agent further serve to create an excellent cathode coating.

BRIEF DESCRIPTION OF THE DRAWING The single FIGURE is a flow diagram of the process.

DESCRIPTION OF THE PREFERRED EMBODIMENTS For a better understanding of the present invention, together with other and further objects,-advantages and capabilities thereof, reference is made to the following disclosure and appended claims.

Referring now to the invention with greater particularity, a thermionic'cathode comprises a substrate of nickel or a nickel alloy containing substantially nickel together with small percentages of one or more reduc: ing agents such as magnesium, silicon and manganese. The total of all the reducing agents is generally less than 6 percent by weight of the alloy. One such material is K3 alloy which is available from GTE Sylvania Incorporated,'Chemica1 and Metallurgical Division, To- Wanda, Pennsylvania. The substrate, for an indirectly heated cathode, is a hollow sleeve which can be of any desired cross-sectional configuration such as circular or rectangular. Applied to the substrate is a layer of nickel particles which is porous, or a nickel alloy such emissive when a reducing agent in the cathode nickel substrate reacts with the oxides to produce an excess of barium in the barium, strontium-calcium-oxygen matrix. The nickel particles and the nickel coated emissive materials are diffusion bonded or welded to each other and to the cathode substrate, the welding also taking place during the aforesaid activation of the cathode. This cathode has excellent adherence of the emissive coating by virtue of the nickel particle layer and the nickel coated emissive material.

The above-described cathode lends itself to a simple and economical fabrication process. A cathode substrate is suitably cleaned by known techniques in preparation for spraying and is placed in a suitable spraying jig, also known in the art.

A suitable nickel particle suspension can be made in the following non-limiting manner.

In a one gallon glass ball mill place 2000 gms of nickel powder (or K3 or other alloy 2300 ml of nitrocellulose lacquer as a binder 470 ml of methyl amyl acetate as a solvent mill the above for hours with 2,560 gms of borundum cylinders and then decant. Rinse the mill with 1,200 ml methyl amyl acetate and add the rinse to the suspension. The suspension is rolled again for 1 hour, without the borundum cylinders, before spraying. The nickel particles can be of conventional configuration or they can be dendritic, as discussed in my application Ser. No. 158,578, filed concurrently herewith.

The potentially emissive materials, which can be mixtures of nickel coated barium and/or strontium and/or calcium carbonates are prepared in a suspension as follows:

In a 1 gallon glass ball mill place 1300 gms of nickel coated carbonates (2 percent nickel by weight) 650 ml of nitrocellulose lacquer as a binder 790 ml of diethyl oxylate as a slow drying solvent 700 ml of diethyl carbonate as a solvent.

In the alternate embodiment 556 ml of barium nitrate suspension as a nickel etching agent is added. The suspension comprises about 160 gms of barium nitrate to 1,600 ml of methyl amyl acetate.

Mill these ingredients for 8 hours.

With both suspensions prepared, each is positioned as a supply source to a separate spraying gun and the process is begun. The previously positioned cathode substrate is now sprayed with a desired thickness of nickel powder. This thickness will vary depending upon the type of cathode being manufactured but generally will not be less than 0.001 inch nor more than 0.005 inch. After spraying with the nickel powder the cathode is sprayed with the carbonate suspension. The nickel particles provide a porous layer on the cathode substrate and the solvents in the cathode suspension carry the carbonates throughout the nickel layer.

If the carbonate suspension is sprayed directly after the nickel suspension, so that the nickel suspension is still wet, then methyl amyl acetate can be used as the solvent in the carbonate suspension. Where, however, there is a possibility that the sprayed nickel layer will have dried, it is preferred to utilize the diethyl oxylate as solvent since it is slower drying than the acetate and thus allows more time for the carbonates to penetrate the nickel layer. After the spraying has been completed, the cathode can be stored until it is ready to be used in a tube. After assembly into a tube the cathode is activated, the nickel particles etched by the action of the nickel etching agent, which is not reactive at room temperature, the solvents and binder volatilized and the nickel particles and the nickel coated emissive materials welded to the cathode substrate and to each other during the final tube processing. During the final processing the cathode temperature is raised from room temperature (22C) to 1,100C. At about approximately the midpoint of this range (592C) the barium nitrate (when used in the alternate embodiment) melts and acts to density the emissive material by carrying it into the nickel layer. As the temperature continuesto rise (at some point over 600C) the [Ba(NO3)2] decomposes according to the equation 2BA(NO )X600C+ 2Ba 4 N0 20 It is believed that the N0 then reacts with water vapor to form HNO The HNO etches the sleeve, the powder nickel layer and the nickel coated emissive materials, probably by forming NiO. The roughened surfaces thus formed increase the bonding capabilities of the various particles and forms a very adherent, dense layer of emissive material. As the processing continues, the NiO is reduced by CO, which is one of the by-products of the carbonate decomposition, to Ni with the CO being oxidized to CO This is then pumped from the tube at exhaust.

The binding phenomenum is called welding since the l,l00C temperature which is reached in final processing is in excess of the normal sintering tempera ture of nickel particles, 975C, but is below the melting temperature which is approximately 1,460C.

The emissive layer produced by this method is extremely tenacious and resists even determined scraping with a knifeblade. Emission characteristics are excellent as are the life of the cathodes'and the resistance to arcing. Excellent heat and electrical conduction is maintained through the nickel particles and the nickel coated emissive materials. 1

While there have been shown and described what are at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.

I claim:

1. A method of making thermionic cathodes which comprises the steps of first forming on a nickel containing substrate a first layer of particles comprised substantially of nickel by spraying said particles in a first volatile suspension from a first spraying source; then spraying 'thereover a potentially emissive material in a second volatile suspension from a second spraying source, said potentially emissive material comprising: nickel coated carbonates of barium and/or strontium and/or calcium, said nickel comprising from 2 to about 7 percent by weight of said carbonates; and subsequently processing said cathode during exhaust at a temperature in excess of the minimum sintering temperature of said nickel particles but below the melting temperature thereof to simultaneously. volatilize said suspensions, weld said nickel particles to said substrate and each other and to said nickel coated carbonates, and activate said emissive material.

spraying thereover a potentially emissive material in a second volatile suspension from a second spraying source, said potentially emissive material comprising nickel coated carbonates of barium and/or strontium and/or calcium, said nickel comprising from 2 to about 7 percent by weight of said carbonates; and subsequently firing said cathode at a temperature above the minimum sintering temperature of said nickel particles but below the. melting temperature thereof.

6. The invention of claim 5 wherein said firing temperature is about 1,100C. 

2. The invention of claim 1 wherein said processing temperature is about 1,100*C.
 3. The invention of claim 2 wherein said second volatile suspension contains a nickel etching agent having an activation temperature of about 600*C.
 4. The invention of claim 3 wherein said etching agent is barium nitrate.
 5. A method of making thermionic cathodes which comprises the steps of first forming on a nickel containing substrate a first layer of particles comprised substantially of nickel by spraying said particles in a first volatile suspension from a first spraying source; then spraying thereover a potentially emissive material in a second volatile suspension from a second spraying source, said potentially emissive material comprising nickel coated carbonates of barium and/or strontium and/oR calcium, said nickel comprising from 2 to about 7 percent by weight of said carbonates; and subsequently firing said cathode at a temperature above the minimum sintering temperature of said nickel particles but below the melting temperature thereof.
 6. The invention of claim 5 wherein said firing temperature is about 1,100*C. 